(a) Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof.
(b) Description of Related Art
A liquid crystal display (LCD) includes an upper panel provided with a common electrode and color filters, a lower panel provided with thin film transistors (TFTs) and pixel electrodes, and a liquid crystal layer interposed between alignment layers of the panels. The LCD displays images by controlling light transmittance, and the control of the light transmittance is performed by applying voltages to the pixel electrodes and the common electrode to generate electric fields which change the arrangement of liquid crystal molecules.
One-dot inversion and two-dot inversion is used for driving the LCD. Both of one-dot and two-dot inversion apply a data signal in a frame having a polarity opposite that of a data signal in a previous frame.
One-dot inversion applies a data signal to a pixel connected to a previous gate line and a data signal to a pixel connected to a present gate line such that the polarity of the two data signals are opposite as shown in FIG. 6A.
Two-dot inversion reverses the polarity of data signals applied to two pixels connected to two gate lines with respect to data signals applied to two pixels connected to previous two gate lines. According to an exemplary two-dot inversion shown in FIG. 6B, if the polarity of a data signal applied to a pixel connected to a current gate line is the same as that of a data signal applied to a pixel connected to a previous gate line, the polarity of a data signal applied to a pixel connected to the next gate line is opposite to that of the data signal applied to the pixel connected to the current gate line.
As the application field of LCDs extends to computer monitors, televisions, etc. which conventional cathode ray tubes (CRTs) have occupied, there occurred the needs for supporting various resolutions and screen scan rates. However, since a conventional LCD has a fixed vertical frequency unlike the CRT, transformations of resolution and scan rate using scale engine and frame memory are required to support various resolutions such as VGA (640×480), SVGA (800×600), XGA (1024×768), SXGA (1280×1024), UXGA (160×1200), etc. and various scan rates such as 60 Hz, 70 Hz, 72 Hz, 75 Hz, 85 Hz, etc.
The recent techniques try to make LCDs support various vertical frequencies by removing frame memory from the LCDs. However, high frequency driving of the LCDs reduces pulse width of gate signals, and the reduction of the gate pulse width in an LCD with the above-described two-dot inversion generates horizontal lines.
In detail, high frequency driving of an LCD results in reduction of the pulse width of the gate signal. If the pulse width of the gate signal is reduced and the load of the data lines is large, a pixel supplied with a data signal having reversed polarity is not sufficiently charged due to the heavy load of the data line. That is, there is unequal charging between pixels connected to the odd-numbered gate lines supplied with the data signal having reversed polarity and those connected to the even-numbered gate lines supplied with the data signal having non-inverted polarity. This charging inequality results in the horizontal line pattern causing poor image quality. This horizontal line pattern also appears in an LCD using 4 mask panel even if it is driven with 60 Hz.
Although it is suggested to use one-dot inversion in the LCD driven with high frequency for avoiding such horizontal line pattern, a dot pattern called flicker is occurred. The flicker is generated when the wave forms of a positive voltage and a negative voltage applied to the liquid crystal are not symmetric. That is, a flicker is a twinkling phenomenon due to the variation of the gray having a period equal to the period of the alternating voltage applied to the pixel electrode because the light transmittance for the positive voltage is different from that for the negative voltage.